Six-ton tires keep cool with grooves

Decatur, IL-When engineers at Caterpillar began designing the new 797B mining truck, they turned to Michelin (Greenville, SC) for help designing tires that will allow the 1.23 million-lb giant carry up to 360 tons per load. "We give Michelin the load specification and they design the tire based on that load," explains Tom Brooks, Caterpillar's design engineer assigned to the project.

In addition to supporting the heavy load on each of the 797B's six tires, Michelin's engineer and project team manager Scott Campbell had concerns about the heat generated by the tires. "The tread and inner casing of the tire generate heat as they roll and go through the deflection cycle under load," explains Campbell. Deflection refers to the vertical flexing of the tire.

"Temperature is a key factor in tire life and performance," he says. Too much heat adversely affects rubber compounds.

The amount of heat generated is a function of the load, tire pressure, and speed. The load and tire pressure determine how much the tire deflects. The speed determines the frequency of the deflection cycle. Therefore, the tire temperature increases as speed increases. With a tire pressure of 100 psi, more than a quarter of a million pounds on each tire, and a top speed of 40 mph, heat was generated faster than the tire could dissipate it with old tread designs.

Campbell explains that engineers from Michelin who worked on the new 59/80R63 XDR tire considered the effects of heat generation when selecting compounds and designing the structure of the tire, particularly the tread. "If we add too much rubber to the tread, the rubber acts like an insulator and keeps the heat in," he notes.

So each new tire has longitudinal and lateral grooves approximately three inches deep. This new tread design includes the additional rubber mass and increased tread depth needed for the big truck's massive weight, without increasing the running temperature of the tire.

The grooves' functions include providing a greater surface area-compared to tires with only longitudinal treads-through which heat is dissipated. The lateral grooves also increase longitudinal adherence when driving and braking. The longitudinal grooves provide lateral adherence during turns.

The criss-crossing grooves also create independent tread blocks that distribute longitudinal forces across the tire's entire tread face. The new design distributes weight equally over the tread, which also helps equalize tread temperatures and prevent hot spots. "We get longer tread life and better protection against shocks and cuts, too," adds Campbell.

Michelin's preliminary estimates indicate that the new tire's tread life is 20% greater than its previous designs.

For more information about heavy-duty tires from Michelin, www.michelin.com/earthmover, or: Enter 538